The use of electrographic processes to produce images is well known. Electrography includes both electrostatic deposition of charge and electrophotography. In the former, an electrostatic charge is produced directly by "spraying" charge onto an accepting dielectric substrate in a controlled manner to generate a latent image graphic.
Styli or "needle electrodes" are often used to create these image patterns and are arranged in linear arrays across the width of the moving dielectric surface. As many as four or five arrays of styli can be used in a "single pass" printer such as those available from Minnesota Mining and Manufacturing Company (3M) of St. Paul, Minn., USA, or one array of styli can be used in a "multiple pass" printer such as those available from Xerox Corporation of Rochester, N.Y., USA.
The latent image electrostatically charged is then developed on the dielectric substrate with suitable toner(s). Usually, at least four colors, cyan, magenta, yellow, and black (CMYK) of toners are employed to generate a myriad of colors through overlapping of toners in any one area of the image. Resolution of images presently exists to 400 dots/inch (dpi).
If the image is developed onto image transfer media, such as 3M's 8601 media, the toner image can then be transferred to a durable substrate. The success of this transfer is supported by the incorporation of a release polymer within or on the dielectric substrate. The incorporation of such a release polymer onto the dielectric substrate increases the susceptibility to scraping or scratching marks due to handling of the toned dielectric substrate. Careful handling of the substrate containing the colorful toner image prior to image transfer is always required.
Scraping or scratching of the image is also a concern during the imaging process within the electrostatic printer, prior to exiting from the printer. The dielectric substrate can be in contact with various areas of the printer such as the styli array, developer rollers, drying rollers, vacuum channels or media transport devices. Physical contact with each of these devices can generate an image scrape of the previously developed toner image, especially if the previous toner color or colors are of high density and/or not thoroughly dried.
The probability for image scraping on image transfer media can be reduced by increase of the total surface roughness. Surface roughness can be measured in Sheffield units and total Sheffield readings above 90 Sheffield Units are preferred in order to minimize the probability of scraping.
Sheffield test procedures and measurements are well defined in TAPPI method T 538 om-88 titled "Smoothness of paper and paperboard (Sheffield method)" published in the year of 1988. Sheffield readings are reported as SCCM [Standard cubic centimeters per minute] or as SHEFFIELD UNITS. There is also an article by George A. Hagerty and John W. Walkinshaw tilted "The Sheffield unit Update to today's technology" published in the January 1988 issue of TAPPI Journal.
For purposes of disclosure of the present invention, the Sheffield Units were direct readings using the Sheffield instrument called Sheffield Surface Measurement Tester--made by Sheffield Measurement Precision Products commercially available from Testing Machines Inc. (TMI) of Amityville, N.Y. USA.
For purposes of the present invention, the term "total surface roughness" refers to reading of the total construction of the dielectric material, not just the roughness of the paper, dielectric layer, or other layers.
For the purposes of the present invention, the terms "scraping" and "scratching" are synonymous.
History has shown an increase in roughness will decrease scraping, but as the roughness is increased usually the transfer efficiency tends to decrease. Roughness is defined for purposes of the present invention as the measurement at the surface which is a total roughness measurement and is influenced by the composite (total) roughness of all the layers within the construction.
History has also demonstrated if a smooth base paper is used then compensation to achieve proper total surface roughness is accomplished by increasing the roughness of the dielectric layer.
In newer high speed electrostatic printers for producing large format full color graphics, such as the 3M Scotchprint.TM. 2000 System from 3M, image drying time is limited from deposition of one toner from a station to the next toner station(s) and there is concern related to image scraping prior to exiting the machine. Even with drying fans operating at maximum capacity, scraping can occur in high speed electrostatic printing systems, especially if more than 4 toner imaging stations exist, such as when a fifth imaging station for "spot colors" or alternative toner or coating compositions.
Sometimes, one desires to generate a black image that comprises all four colors, yielding a very high density of toner on a given area of the dielectric substrate. In the case of a solid four color black image, the printing speed of the system must be reduced to less than the maximum value in order to give more drying time, in order to minimize scraping of some of the printed surface at the fifth station. Alternatively, voltage contrast must be reduced in order to limit toner density. As a last resort, the fifth station must be removed from the printer.
Again, this concern related to scratching the image prior to exiting of either single pass or multiple pass printers arises from the incorporation of release polymer near the image surface of the dielectric substrate. After all, the deposition of that toned image is for transfer, but not prematurely through scraping within the printer itself.